In the manufacture of semiconductor wafers and of other similarly manufactured articles, sequences of processes including coating, etching, heat treating and patterning are sequentially employed. Most of these processes involve the chemical or physical addition or removal of material to or from a surface of a substrate, usually transported as a vapor.
Certain coating processes in such sequences are performed by chemical vapor deposition (CVD). CVD is preferred, for example, in applying films to the differently facing surfaces of holes through underlying layers, as, for example, to apply conductive films for the purpose of making interconnections across insulating layers and the like.
The ultimate result of CVD processes for filling holes or vias, and for forming interconnections between layers on semiconductor wafers, is frequently the selective deposition of the film, that is, formation of a permanent film on only selected portions of the wafer surfaces. Direct selective application by CVD of such coatings is often unreliable, unsuccessful, or slow, and thus undesirable on a commercial scale, where rapid throughput and efficient use of expensive machinery is important. Therefore, selective end product films are often applied in blanket fashion and then etched back from the areas where permanent film is undesired.
Blanket CVD of materials, such as tungsten, followed by an etching back of the deposited material, requires a high degree of uniformity in the blanket film, particularly on the areas of a substrate from which the material is to be etched. If the coating is irregular in the etch-back areas, the etching process may selectively damage the underlying layers in regions of the wafer where the blanket film to be etched is thin, or may result in regions where residual film remains. CVD reactors of the prior art have coated substrates with limited uniformity, or at limited speed. Accordingly, more uniform application of the films and higher speed CVD reactors, particularly for blanket coating applications of materials such as tungsten, are required.
To uniformly apply films such as tungsten by CVD to semiconductor wafers, it is desirable to ensure a uniform supply of reactant gases across the surfaces of the wafers, and to uniformly remove spent gases and reaction byproducts from the surfaces being coated. In this respect, prior art CVD reactors perform with limited success. Similarly, in other processes such as physical and chemical etching and heat treating processes, including preheating and annealing processes, prior art systems have been inadequate in uniformly bringing vapors into contact with, and removing them from, the surface being processed. Accordingly, there is a need to more efficiently and more uniformly supply and remove reaction and other gases to and from the surfaces of wafers being processed, and particularly those being coated by CVD processes.
Efficient commercial production of semiconductor wafers requires that the processing equipment function as continuously as possible. When deposits form on interior components of processing chambers, such as those of CVD reactors, they become ineffective and their use must be suspended for cleaning. Many reactors of the prior art require cleaning at an undesirable frequency, or are too difficult and too slow to clean, thus resulting in excessive reactor downtime. Accordingly, there is a continuing need for processing chambers such as those of CVD reactors that require less frequent cleaning of components, that reduce unwanted deposition on components, and that can be cleaned more rapidly.
In the chambers of CVD reactors and other wafer processors of the prior art, turbulence in the flow of reaction gases has inhibited the efficiency and uniformity of the coating process and has aggravated the deposition and migration of contaminants within the reaction chamber. Accordingly, there is a need for improved gas flow, and reduced gas flow turbulence, within such chambers.
CVD processes such as those for the application of tungsten coatings to semiconductor wafers are typically performed in cold wall reactors, where the wafers to be coated are heated to a reaction temperature on a susceptor while other surfaces of the reactor are maintained at subreaction temperatures to prevent the deposition of films thereon. For tungsten CVD, for example, reactor walls are often cooled, often to about room temperature. Alternatively, for titanium nitride (TiN) CVD, the walls may be heated above room temperature, but to a temperature below that of the substrate being treated. In such cases, there is a need in the designs of such wafer processing devices that have components that are maintained at different temperatures to prevent heat from flowing between the wafer or susceptor and other components of the apparatus.
In tungsten CVD processes, tungsten hexafluoride gas (WF.sub.6) is commonly employed. This WF.sub.6 gas is costly, as are the gases employed in many other wafer treating processes. When the gas utilization efficiency is low, as is the case of many reactors of the prior art, the cost of the gas can be high. With many tungsten CVD reactors, the utilization efficiency of WF.sub.6 is below twenty percent, and the cost of the WF.sub.6 often exceeds thirty percent of the entire cost of the performance of the process for application of the tungsten film. Accordingly, CVD reactors that are more efficient in the consumption of reactant gases such as WF.sub.6 are required.
CVD processes may be divided into two catagories, those that are mass transport controlled and those that are surface condition or temperature controlled. Mass transport controlled processes are typically those involving the CVD of group III-V materials onto substrates such as gallium arsenide wafers or for the epitaxial growth of silicon. Such processes are controlled by the transport of gases to and from the wafer surfaces and have been used by moving the wafers, typically mounted in pluralities on rotating or otherwise moving susceptors that cause the substrates to orbit about an axis in a flowing gas, or otherwise employing techniques to enhance and control the gas flow across the wafers. Typically, the mass transport controlled CVD processes will be found on an Arrhenius plot, that is a plot of the log of the deposition rate versus the reciprocal of the temperature, above the knee in the curve.
Wafer temperature or surface condition controlled CVD processes are typically found below the knee of the Arrhenius plot curve. These are brought about by higher temperatures, and usually at lower pressures of from 1 to 100 Torr. Generally, such processes are not regarded in the prior art as amenable to enhancement by wafer movement, except to achieve temperature or reaction uniformity, which is promoted with low speed movement.